In association with the trend toward smaller-sized semiconductor devices and electrical/electronic components, metal particles having a particle diameter of not more than 100 nm, so-called metal nanoparticles, have attracted attention for their applicability to semiconductor devices, etc. Examples of the application of metal nanoparticles to semiconductor devices, etc. include the formation of interconnects with a small amount of a liquid containing metal nanoparticles and the use of a conductive paste containing metal nanoparticles, for example.
Sn—Pb solders, which have been widely used for bonding of various devices to a substrate in a semiconductor device mounting process, for bonding between electrodes of a high-power semiconductor device, etc., are required to be replaced with lead-free solders from the viewpoint of environmental conservation. A technique is almost established to replace a low-temperature solder, such as a common 60% Sn-40% Pb solder, of Sn—Pb solders chiefly with a lead-free Sn—Ag—Cu solder. For a Sn-95% Pb high-temperature solder having a melting point of about 300° C., however, there is at present no prospect of a substitute solder material of lead-free composition. It would therefore be very advantageous if the use of Sn-95% Pb high-temperature solder could be abolished totally by replacing the high-temperature solder with a bonding material comprising as a base material composite nanoparticles each composed of a metal nanoparticle or an inorganic metal compound nanoparticle as a core, and carrying out bonding based on the low-temperature sintering property of the nanoparticles.
Metal nanoparticles are known to generally take on properties different from the bulk metal material as the particle diameter decreases. This is considered to be due to the fact that the proportion of the atoms exposed on a surface of a metal nanoparticle to all atoms contained in the nanoparticle is much higher compared to the bulk metal. One of typical properties of metal nanoparticles is a temperature at which sintering occurs. Table 1 shows sintering initiation temperatures of various types of metal nanoparticles having a particle size of about 20 to 50 nm (see Ichinose, Ozaki and Gasyu, “Approach to Ultrafine Particle Technology”, Ohmsha, Ltd., July 1988).
TABLE 1SinteringMetalDiameter (nm)initiation temp. (° C.)Fe50300-400Ag2060-80Ni20-200Cu200
As is apparent from Table 1, metal nanoparticles initiate sintering at remarkably lower temperatures as compared to particles commonly used industrially. There is a strong likelihood that by utilizing such low-temperature sintering property of metal nanoparticles and applying metal nanoparticles to low-temperature bonding of members, metal nanoparticles will replace lead-containing solders commonly used as a bonding material for electric components, semiconductor devices, etc.
On the other hand, metal nanoparticles generally have a very high surface activity and, therefore, tend to attract each other, so that the particles get closer to each other and agglomerate even at room temperature. Once metal nanoparticles agglomerate into coarse particles, the unique properties of nanoparticles are instantly lost. For this reason, it has generally been considered difficult to apply metal nanoparticles to, for example, the formation of fine interconnects or filling-in of vias of very small size in a semiconductor.
In view of this, composite metal nanoparticles, in which each metal nanoparticle is covered with a coating of an organic substance to protect the metal nanoparticle, have been developed. The following two methods are generally known to coat and protect a surface of a metal nanoparticle with an organic substance: (1) a method of forming a solvent coating film on a surface of each metal nanoparticle in the course of the formation of the metal nanoparticle by a physical means and before the particles collide with each other to agglomerate; and (2) a method of allowing a solvent, a metal salt, a protective agent, a reducing agent, etc. to coexist in a liquid-phase system, and heating the system.
The method (1), which involves the formation of metal nanoparticles by a physical means, necessitates vaporization of a starting metal principally in a gas, which is likely to lead to a low-productivity costly process. The liquid-phase method (2), which involves the formation of composite metal nanoparticles with a liquefied starting material for the particles under atmospheric pressure, is advantageous over the method (1) in that a low-cost mass productive process can be easily established.
A method belonging to the method (2) has been proposed which comprises heating a starting material, for example, silver stearate at 250° C. in a nitrogen gas atmosphere to produce composite silver nanoparticles (see, for example, Japanese Patent Laid-Open Publication No. H10-183207). This method can produce a composite silver nanoparticle 20, as shown in FIG. 1, comprising a metal core 22 of a metal (silver) component, having an average particle diameter d2 of, e.g., about 5 nm, and a coating of an organic substance 24, having a thickness h2 of, e.g., about 1.5 nm. It has been confirmed that a temperature of at least 250° C. is necessary to separate the organic substance 24′ from the surface of the metal core 22 (metal component) of the composite silver nanoparticle 20 and uniformly sinter the metal cores 22, and that the bonding temperature of a bonding material comprising the composite silver nanoparticles 20 as a base material is also at least 250° C. It is considered in this regard that the organic substance 24 is chemically bonded to the surface of the metal core 22, that is, an organometallic compound with the metal (silver) taken in has been formed by a reaction between the organic substance 24 and the metal core 22, and therefore the bonding energy is so high that the high temperature (energy) is necessary to release the bonding and separate the organic substance 24 from the metal core 22.
It has also been proposed to produce composite silver nanoparticles by allowing a metal salt and an alcoholic organic material to coexist, and heating the mixture at a temperature equal to or higher than the decomposition initiation temperature of the alcoholic organic material (see, for example, International Publication WO 01/70435 Pamphlet). This method can produce a composite silver nanoparticle comprising a metal core of a metal (silver) component, having an average particle diameter of, e.g., about 7 to 10 nm, and a coating of an organic substance, having a thickness of, e.g., about 1.5 nm. It has been confirmed that, as with the above-described composite silver nanoparticles, a temperature of at least 250° C. is necessary to separate the organic substance from the surface of the metal core of this composite silver nanoparticle and uniformly sinter the metal cores, and that the bonding temperature of a bonding material comprising the composite silver nanoparticles as a base material is also at least 250° C. This is considered to be due to chemical bonding of the organic substance to the metal core through the formation of an organometallic compound as in the above-described case.